Leetcode 3212. Count Submatrices With Equal Frequency of X and Y

Count the number of axis-aligned submatrices in a 2D grid of 'X', 'Y', and '.' that contain at least one 'X' and have an equal number of 'X' and 'Y' ('.' cells are ignored); grid sizes can be up to 1000×1000 so an efficient solution is required.